It’s, pretty straightforward. From Zermatt you take the Matterhorn Express gondola up to Trockener Steg (P1, P2, P3, P4). It passes through several lift stations on the way, so unless told otherwise by the staff stay on it until you get to Trockener Steg, from there take the Matterhorn Glacier Ride tricable gondola (3S) up to Klein Matterhorn. The cable cars (O1, O2, O3) shown on the piste map aren’t in regular operation, they are mainly used for personnel / goods transport.
There are 2 routes from Klein Matterhorn over to Cervinia. The 1st route goes via Testa Grigia, the decent to Cervinia is via piste 7, the famous Ventina piste. By the time you go it may be possible to ski down a section of the new downhill course, there is an access track from Testa Grigia, the top section of the Ventina runs diagonally across the slope so isn’t very pleasant to ski.
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The second route is via the Theodul Pass, if I were you I would take the 1st route.
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There are a few things that you should be aware of. The glacier is heavily crevassed, so you should stay on the marked runs. The area is subject to high winds, especially in the early part of the season, so the link might not be open. You should take your passport and insurance documents with you, 3rd county citizens are required to carry their passports at all times in Italy and Italy requires that you have 3rd party insurance for skiing.
Give some thought to getting back to Zermatt, don’t leave it too late! I usually take the chairlifts from Plan Maison to the Theodul Pass to get back.
There is an interactive piste map on the Zermatt lift company website
https://www.matterhornparadise.ch/en/Experience/Zermatt-ski-area/Ski-map and a pdf version on the Cervinia website
https://cervinia-api.therocks.it/wp-content/uploads/2021/10/CerviniaVlt-SkiramaInverno-ENG-1.pdf